The disclosed subject matter relates to composite filter media. In particular, the disclosed subject matter relates to composite filter media comprising multiple layers of laminated media material including a nanofiber media layer.
Applications, such as on-board engine high pressure common rail fuel filter systems, are demanding. High levels of particle removal for fine particles should be maintained even under conditions of flow surges and vibration, as are encountered on operating engines. Particles 4 μm (c) or even smaller have been shown to damage fuel injectors, and beta ratios in excess of 64, and ideally higher are recommended. Further, these levels of removal should be maintained even when flow rates change rapidly, such as at engine start up or when the fuel throttle is rapidly adjusted, or when the engine vibrates. At the same time, customers and end-users insist that filter life not be shortened, while space constraints prevent the use of larger filters.
These challenges are further exacerbated in typical diesel fuel applications, where multiple filters are used in series. Multiple filters are typically used on modern diesel fuel systems. In many cases, a first filter FWS (Fuel Water Separator) or coalescing filter is used both to remove water and pre-filter coarser particles. This first filter or pre-filter results in a significant shift in contaminant particle size distribution and may adversely affect contaminant removal of a subsequent filter due to the removal of the coarser particles. Coarser particles tend to block the larger pores in a typical media used in these systems. The absence or reduced concentration of these coarser particles allows larger pores in the downstream filter or filter media to remain open to the flow, which in turn allows contaminant to pass through. This results in depressed removal in this size range.
In the past, the aforementioned challenges could be addressed by means of composite media with layers of media with relatively broad pore size distributions that provided graded particle capture, i.e., successive layers capturing nominally finer particles. However, much higher levels of contaminant removal for much finer particles are now required under transient conditions and highly efficient nanofibers are now required to achieve these levels of removal. This may result in shortened service life.